For starters, if you don't have a bike, the best time is always now. Seriously, get out there it's a blast.
But if you have a bike and are just looking to upgrade, then you have twice the selection when the new models start to ship. You can choose to get the latest and greatest, or get a great deal on last year's model -- the best deal isn't always the cheapest. Not only can this save you some money, but it can also save you regret if the new model actually features something you really valued. No matter what you choose to do, become an expert on it. Make sure you are familiar with the differences in the models and how they match up with what you like to ride -- if you are new to the sport, ask shops about demos and rentals. Even a used bike can be a bad deal if you don't know how to check it out in order avoid costly repairs -- do the fork seals need replacing, etc.
If you are just starting to learn about mountain bikes, and can't ride right away for health reasons, you might wait until the Fat Tire Festival at Alafia/Balm on Nov 9th-10th. There will be lots of demos trailers there with every bike under the sun. It would be a great chance to test out different models from different manufacturers, back to back on the same trail.